Positive displacement compressors are machines in which successive volumes of air or gas are confined within a closed space and elevated to a higher pressure. The pressure of the gas is increased while the volume of the closed space is decreased. Positive displacement compressors include, for example, reciprocating compressors, rotary compressors, scroll compressors and screw compressors. Screw compressors, also known as helical lobe rotary compressors, including single screw compressors, male-female (double) screw compressors and other variations, are well-known in the air compressor, refrigeration, water chiller, and natural gas processing industries. These compressors rely on lubricating oil to lubricate rotating and contacting surfaces to allow for efficient operation, to prevent damage to the units and to seal the lobes containing the volume being compressed.
Reciprocating compressors utilize a movable piston in a cylinder. The piston is attached to a connecting rod which is attached to a crank. An electric motor drives the crank which causes the piston to reciprocate within the cylinder, increasing and decreasing the volume within the cylinder. Fluid is introduced into the cylinder through a valve when the piston is at the bottom of its stroke. The fluid is compressed as the piston moves toward the top of its stroke and is removed from the cylinder through a valve when the piston is at the top dead center (TDC) its stroke. Lubricant is utilized to lubricate the bearings, the cylinder walls, piston walls, piston rings, if utilized, and piston pins. Smaller reciprocating compressors are usually sealed units and entrainment of lubricant in the compressed refrigerant usually is not a problem. However, for larger reciprocating compressors, lubrication loss can present a problem.
A scroll compressor generates a series of crescent-shaped pockets between two scrolls, the crescent-shaped pockets receiving fluid for compression. Typically, one scroll is fixed and the other orbits around the fixed scroll. As the motion occurs, the pockets between the two forms are slowly pushed to the center of the two scrolls. This reduces the fluid volume. Lubrication is used to lubricate the main bearings and seal surfaces along and at the edge of the scrolls.
Rotary compressors are of two general types: stationary blade and rotating blade compressors. The blades or vanes on a rotating blade rotary compressor rotate with the shaft within a cylindrical housing. In a stationary blade compressor, the stationary blade has a blade that remains stationary and is part of the housing assembly, while a cylinder rotates within the housing assembly, via a roller on an eccentric shaft within the cylinder. In both types, the blade provides a continuous seal for the fluid. Low pressure fluid from a suction line is drawn into an opening. The fluid fills the space behind the blade as it revolves. The trapped fluid in the vapor space ahead of the blade is compressed until it can be pushed into the compressor exhaust. A film of lubricant is required on the cylinder, the housing, roller and blade surfaces as well as on the bearings. Lubricant can readily become entrained in the refrigerant.
A screw compressor generally includes two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The side walls of the compressor casing typically form two parallel, overlapping cylinders which house the rotors side-by-side, with their shafts parallel to the ground. Screw compressor rotors typically have helically extending lobes and grooves on their outer surfaces forming a large thread on the circumference of the rotor. During operation, the threads of the rotors mesh together, with the lobes on one rotor meshing with the corresponding grooves on the other rotor to form a series of gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or “port,” at one end of the casing and continuously reduces in volume as the rotors turn and compress the gas toward a discharge port at the opposite end of the casing. Lubricant is introduced into the compressor to lubricate the bearings, shaft seal and rotors, to help seal the clearances between the screws during operation of the compressor, to help remove the heat of compression thereby preventing the lubricant from overheating and to help reduce the noise associated with compressor operation.
Common to each type of compressor is an inlet and an outlet. A compressor inlet is sometimes also referred to as the “suction” or “low pressure side,” while the discharge is referred to as the “outlet” or “high pressure side.”
Screw compressor rotors intermesh with one another and rotate in opposite directions in synchronization within a housing. The rotors operate to sweep a gas through the housing from an intake manifold at one end of the housing to an output manifold at the other end of the housing. Commercially available screw compressors most commonly include threaded shafts or helical rotors having four lobes, however, others have been designed to have five or more lobes, and rotors may have any number of lobes, for example from 3–9 lobes. Male and female rotors typically have different number of lobes. The rotor shafts are typically supported at the end walls of the casing by lubricated bearings and/or seals that receive a constant supply of lubricant from a lubricant circulation system.
Lubricants typically are some type of oil-based liquid compound, this part of the compressor system often being referred to simply as the “lube-oil” system. Compressor lube-oil systems generally include a collection reservoir, filter, and pressure and/or temperature sensors. The lube-oil may be circulated as a result of the pressure differential in the system across the evaporator and condenser, such as in water chiller screw drive compressor system, or the lube-oil may be circulated by a motor driven pump such as in larger reciprocating compressors. Since many lubricants degrade at high temperature by losing “viscosity,” compressors operating at high temperatures, such as with screw compressors, generally include specially formulated lube-oil systems and also include a cooler for reducing the temperature of the lubricant before it is recirculated to the seals and bearings. So-called “oil flooded” screw compressors further include means for recirculating lubricant through the inside of the compressor casing. Such “lube-oil injection” directly into the gas stream has been found to help cool and lubricate the rotors, block gas leakage paths between or around the rotors, inhibit corrosion, and minimize the level of noise produced by screw compressors.
As is evident in these positive displacement type compressors, lubricant and fluid in the gaseous state being compressed together, are mixed as a result of compressor operation. Under these high pressures and temperatures, the lubricant forms droplets. These droplets typically are entrained in the gas stream and must be removed before the compressed gas, typically a refrigerant, is transported away from the compressor. When these droplets are very fine, typically smaller than about 1 micron, they form an aerosol which is entrained within the refrigerant gas. These aerosols do not readily coalesce and are not readily removable from the fluid stream without inclusion of special aerosol-removing equipment such as coalescers, which is a part of a separator portion of a compressor system. In one alternative for closed systems, the aerosols can be allowed to move downstream with the compressed gas, since the aerosol will eventually be returned to lubricate the compressor. Of course, this requires additional lubricant in the system to accommodate the volume of lubricant that is always absent from the compressor, which adds to the cost of operation since lubricant is expensive. Furthermore, the presence of the lubricant in other parts of a closed system can lead to the downstream equipment not operating efficiently. For open systems, the lubricant is lost downstream as aerosol which is never returned and must be replenished.
A typical screw compressor mixes together lubricant and refrigerant, discharging a high-pressure and high-temperature fluid stream consisting of a mixture of compressed gas and oil. The oil at the high temperatures and pressures will form droplets across the size range set forth above, including in the aerosol range, 1 micron and smaller. Without aid, the entrained aerosol does not tend to coalesce and form droplets that can be removed easily from the compressed gas. As a result, coalescers are included in separators to remove the aerosol to prevent the lubricant from being carried downstream of the compressor. Even with coalescers, a very small amount of exceedingly fine droplets escapes downstream. If a sufficient volume of oil is removed from the compressor, the compressor undesirably can be depleted of oil.
For designs in which excess oil is not utilized, the oil must be kept within the compressor. The oil must be separated from the high pressure refrigerant gas before the refrigerant gas is discharged into the chiller or refrigeration system, which entails the agglomeration of the finely-divided aerosol.
As noted above, to prevent the lubricant from being entrained in a fluid moving downstream, the prior art employs a compressor having a separator section. The compressed gas may be forced to follow a tortuous path or contact a surface where larger droplets can agglomerate and can be cycled back into a sump-type device for reuse, lubricating the moving parts of the compressor. To capture the finer aerosol, that are not agglomerated into droplets of sufficient size to be separated, the separator section may employ a coalescer or filter unit through which the aerosol must pass before discharge of the compressed gas downstream of the separator. While these designs are effective in agglomerating the oil and minimizing the loss of oil to the chiller system, the compressed fluid undergoes a pressure loss as the mixture of aerosol, which resembles a smoke, passes through the coalescing device. This pressure loss is directly related to system performance, reducing the efficiency of the unit. A typical coalescer element comprises a series of filters providing high surface area made of mesh microfiber materials or system filters, which also increase the size of the system. In applications where space is not a consideration, the size of the system is not an important factor, but pressure drop remains a concern, although a larger vessel can be placed in the same volume of space. However, in most applications, space is a consideration, and the separator occupies space that could be otherwise utilized. Elimination of the coalescer elements would permit installation of either a larger separator within the same space, allowing for larger compressor systems, or systems can be designed with the same capacity, but less space. In addition, the efficiency can be improved as the pressure drop associated with coalescers can be removed from the system. Compounding this situation, units that include a coalescer element typically provide access to the interior so that internal filters and passageways can be maintained. Such access, usually provided through a manway, requires yet additional space for access. In addition, the access requires an additional penetration into the system that must be appropriately sealed with a suitable gasket. However, this gasketed joint undesirably provides a potential leak path. A further disadvantage is the additional cost associated with manufacturing the structures that house the coalescer element.
A method that has been suggested to eliminate the coalescer is the use of non-smoking lubricants. The term non-smoking lubricant means a lubricant that does not form an aerosol, or, in the alternative, one that forms an aerosol whose submicron-sized and micron-sized particles exist for a very short time and which can be readily manipulated to coalesce into droplets of sufficient size that can easily be segregated from the compressed gas within the working volume of a separator without the use of a coalescer. These non-smoking lubricants have found application as machining/cutting oils to minimize the inhalation exposure of machine operators to such oils. While these lubricants have been proposed for use with compressor systems, none of the lubricants has found commercial application for use with refrigerants typically used in compressor systems. These lubricants also tend to be more expensive, and their use in existing positive-displacement type compressor systems, such as screw compressor systems have provided no distinct advantages to justify their increased cost.
One lubricant that has been proposed for use is set forth in U.S. Pat. No. 3,805,018 which sets forth a stray mist suppressant that includes oil-soluble polyolefins of viscosity average molecular weight greater than 5,000. Another is set forth in U.S. Pat. No. 5,756,430 which teaches a mist oil lubricant based an polycarboxylic acid ester to which is added 1–5% polyisobutylene Mn 400–2500 as a stray mist suppressant. While both of these teach the formulations of potential smokeless oils, neither of these recognize the full potential for enhanced system performance and equipment improvement resulting from the use of such a smokeless oil except as a direct substitute for existing lubricants in existing systems.
Similarly, U.S. Pat. Nos. 4,916,914 and 5,027,606 to Short and assigned to CPI Engineering Services, Inc. disclose the use of a lubricant that will not readily dissolve in refrigerant at higher temperatures and pressures, but will readily dissolve in refrigerant at low temperatures and pressures. This can be accomplished by providing a refrigerant-lubricant combination at which there are substantially two phases at condensing temperatures and pressures, and, substantially one phase at evaporation temperatures and pressures. This combination permits the oil to be more efficiently separated from the refrigerant in the discharge region before the fluid is discharged into the chiller system downstream of the compressor. However, the patents do not recognize system improvements that can be engineered as a result of the use of such lubricants. These lubricants are polyether polyols or monols in combination with a non-chlorinated hydrocarbon refrigerant.
Present compression systems include separators used in conjunction with the compressors. The separators function to separate lubricant from refrigerant and have elements which perform the same functions, regardless of the type of lubricant that is utilized in the device. These separators have been designed for use with lubricants that mix with refrigerant gas to form a fluid that includes fine aerosols or “smoke” as they are discharged from the compressor. Current compression systems route the fluid from the compressor portion of the compressor through a small pipe into a large pipe, which typically discharges within the separator. As the fluid is discharged from the small pipe into a large pipe, there is a velocity change. The fluid impinges a wall of the separator and undergoes a change of direction, and again loses some velocity. With each surface that the fluid contacts, there is some energy loss and some coalescence of the lubricant droplets across the size spectrum of droplets. When the coalescing lubricant reaches a critical size, it separates from the refrigerant gas by gravity or momentum and drops to the bottom of the separator, which forms a main oil reservoir supply for the compressor, being cycled back from the separator to provide lubrication to the compressor. The remaining fluid passes through structure within the separator referred to as the coalescer element, where a substantial portion of the remaining aerosol coalesces into droplets on the fine material fiber with increased surface area, after which, by gravity, it falls into a coalescer reservoir associated with the coalescer element that is maintained at a lower pressure than the main oil reservoir. Oil is returned from the coalescer reservoir to the low pressure side of the compressor by a separate line. Fluid passing through the coalescer element then exits the separator and passes downstream of the compressor, which for a closed system entails passing into the remaining portion of the compression system. This fluid includes refrigerant and still may include a small amount of lubricant as very fine aerosol which has been able to pass through even the fine filter elements of the coalescer. Access to the internal mechanisms of the separator, when provided, is through a manway typically located in a head at one end of the separator adjacent the coalescer. This access is required as the coalescer elements may require periodic maintenance and replacement.
However, none of the prior art discloses the advantages that can be realized in the chiller systems by incorporation of the smokeless-type oils into the chiller systems. Smokeless lubricants, such as lubricants being developed by CPI, Inc of Midland, Mich., are described in copending patent application entitled A LUBRICANT AND COMPRESSOR WORKING FLUID COMPOSITION USEFUL FOR IMPROVING THE OIL SEPARATION PERFORMANCE OF A VAPOR COMPRESSION SYSTEM, assigned to Lubrizol Corp. of Cleveland, Ohio., filed on the same day as the present application. What is needed are compressor systems that incorporate modified structures with reduced sizes as a result of the use of smokeless type oils.